The present invention relates to a thermal overload relay (thermal relay) used in combination with an electromagnetic contactor and the like, and more particularly, to a retaining structure of an adjustment dial thereof.
An explanation is first provided for a conventional structure of the above-mentioned thermal overload relay with reference to FIG. 12. In FIG. 12, reference symbol 1 indicates a molded plastic external case, 2 a main bimetal that is bent by heat generated by a heater connected to a main circuit, 3 a shifter coupled to the free end of the main bimetal 2, 4 a releasing lever driven by displacement of the shifter 3, 5 an inversion operation mechanism for opening or closing a contact, 6 a normally closed contact comprising a movable contact 6a and a fixed contact 6b, 7 a normally open contact comprising a fixed contact 7a and a movable contact 7b provided on a movable contact shoe piece, 8 an adjustment link having a lower end supported in a case slot 1a provided in the external case 1 and being coupled to a proximal end of the releasing lever 4 at a support point 4a, and 9 an adjustment dial comprising a head section 9c (FIG. 13) and a shaft section 9g and having a cam surface 9a on the lower end of the shaft section 9g, an upper end 8a of the adjustment link 8 contacting the cam surface 9a. 
Furthermore, a current adjustment hole 9d is a hole for inserting a screwdriver for adjusting settling current, and current setting scale 9e is a scale for aligning with an indicated value provided on the external case 1. Here, the inversion operation mechanism 5 comprises a pivotally movable plate 11 having one end locked and supported in a V-shaped groove 10a of a generally U-shaped support piece 10, a tension spring 12 extending between a distal end section 11a of the movable plate 11 and a spring catching section 10b of the support piece 10, and a normally open contact drive lever 13 protruding towards the back from the movable plate 11 in the shape of the letter L, and the movable contact 6a of the normally closed contact 6 is attached to the distal end section of the movable plate 11. In addition, the fixed contact 6b of the normally closed contact 6 has one end fixed to the bottom of the external case 1 and is attached to a contact supporting piece 14 having a flat spring structure lying in the horizontal direction.
The tension spring 12 has a coil-like spring section 12a formed of a wire of a spring steel material and a protruding section 12b formed to protrude into the center of the tensile spring 12, and has hook sections formed on both ends thereof. With this configuration, in the steady state shown in FIG. 12, the movable plate 11 of the inversion operation mechanism 5 tilts in the clockwise direction from its neutral position due to spring force applied from the tension spring 12, and the movable contact 6a of the normally closed contact 6 presses against the fixed contact 6b to maintain the contact in the “on” state. Furthermore, the normally open contact 7 is “off” in this state.
Here, when an overload current flows through a main circuit, the main bimetal 2 is heated and bent, and the shifter 3 moves to the right due to displacement of the free end thereof. As a result, releasing lever 4 pivots around a support point 4a. At this time, in addition to the projecting section 12b of the tension spring 12 of the inversion operation mechanism 5 being pushed upward, when the displacement of the tension spring 12 exceeds a dead point of the movable plate 11, the movable plate 11 is rapidly driven to be inverted, and together with the movable contact 6a of the normally closed contact 6 separating from the fixed contact 6b, a movable contact show piece to which movable contact 7b is attached is pressed by the drive lever 13, and the movable contact 7b contacts the fixed contact 7a to turn on the contact (see, for example, Patent document 1).
Next, an explanation is provided for a method for adjusting a settling current value of a thermal overload relay based on FIG. 12. In FIG. 12, when the adjustment dial 9 is rotated, the adjustment link 8, having the upper end 8a contacting the cam surface 9a, is displaced by using the case slot 1a of the external case 1 as a support point. Accompanying this, the releasing lever 4 coupled to the adjustment link 8 can be displaced and moved to change the gap between the releasing lever 4 and a distal end of the shifter 3.
Here, an explanation is provided for a method of retaining the adjusting dial 9 in an adjustment dial insertion hole 1b provided in the external case 1 based on FIG. 13. In FIG. 13, in order to retain the adjustment dial 9 at a prescribed incorporated position, a wire spring 15, obtained by bending a spring wire material (such as a piano wire) to a shape like that shown in FIG. 13, is disposed as shown in FIG. 10, and the adjustment dial 9 is pressed and held at a fixed position by the spring reaction force thereof. In addition, Patent document 2 discloses a configuration in which an adjustment dial is retained by a handle linking bracket and a pressing spring.
Patent document 1: Japanese Patent Application Laid-open No. 2001-160346, corresponding U.S. Pat. No. 6,459,355 B1
Patent document 2: Japanese Utility Model Application Laid-open No. S53-95168
However, in a thermal overload relay of the prior art as described above, a wire spring 15 or other part is required to retain the adjustment dial 9 at a fixed position, thereby resulting in the problems of increasing the number of parts and number of assembly steps during product assembly as well as increasing costs.
With the foregoing, an object of the present invention is to provide a thermal overload relay having an improved structure for retaining the adjustment dial 9 so as to enable the adjustment dial 9 to be stably retained in the external case 1 without increasing the number of parts.
Further objects and advantages of the invention will be apparent from the following description of the invention.